The invention relates to a method of making a dendritic web of silicon forming a continuous ribbon consisting of a single silicon crystal and more particularly to a method of inhibiting the formation of dislocations in the dendrites adjacent the web.
Prediction and control of dislocation generation during crystal growth is a key task in defining attainable regimes of ribbon growth rate and solar cell efficiency. This aspect of materials engineering poses a formidable challenge since no complete field theory for dislocation source operation and defect accumulation or annihilation exists at present. Consequently, semi-empirical and phenomenological models must be constructed.
A qualitative model, which appears to be consistent with most of the data on dislocation density, distribution, Burger's vector, and residual stress is as follows: dislocation sources located in the bounding dendrites or near the external {111} surface of the web generate dislocations when thermal stresses exceed approximately 2 MPa over a distance of zero to approximately 3 cm from the growth front. Dislocation generation may also occur in sections of the ribbon lying within 4 to 7 cm from the growth front, if larger critical resolved shear stresses exist before the crystal has cooled below approximately 1000.degree. K. Most of the dislocations glide into the web until they encounter the internal twin boundaries or a portion of the liquid-solid interface. A twin boundary is a barrier to dislocation motion and results in dislocation pile-ups. These can undergo thermal rearrangement to form low angle polygonal boundaries or higher angle subgrain boundaries which usually terminate crystal growth and reduce solar cell efficiency. Dislocations which intersect the solid-liquid interface are subsequently propagated as growth steps as atoms attached to the growing web. These grown-in dislocations are aligned along the [211] growth direction and are usually heavily decorated with SiO.sub.x precipitate particles.
X-ray transmission topographs indicate that dislocation sources in the bounding dendrites can inject segments of expanding dislocation loops into a silicon web crystal during growth. Darkly imaging features near the outside edge of the dendrites have been identified as local stress centers by a variety of techniques including Sirtl etch pitting of bevel polished dendrites, X-ray diffraction analysis, and transmission electron microscopy observations of bend contours associated with these features. The occurrence of similar stress centers in as grown web crystals was reported in the early days of web research by S. O'Hara, in his paper entitled "Dislocation in Webs of Germanium and Silicon" and in T. N. Tucker and G. H. Schwuttke in their paper entitled "Growth of Dislocation-Free Silicon Web Crystals."